R975 Radial engines

[maple_leaf_eh]

Originally Posted by Bob Phillips

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Some of the cylinders were reconditioned by the Van der Horst Corporation. These cylinders were reconditioned by honing the used bores out oversize, and then chroming the bores back to standard size. This technology had been developed through a patented process by a Dutch engineer named Dr Hendrik Van der Horst in 1936. He called his patented process "porous chrome". He improved his chrome plating process again in 1943 and set up a plating company in the USA called Van der Horst Corporation of America ( later United Van der Horst). His challenge had been to create a plating process that was not as hard and smooth as traditional chrome plating. The chrome needed to have the ability to retain oil in it's surface. This was accomplished by chroming the bores in the traditional way but then near the end of the process to reverse the polarity of the electrical charge which attracted the chromium solution onto the bore surface and in so doing, create a series of tiny grooves and furrows which would retain the lubricating oil. The finished bore has a frosted appearance but is still very hard. ...

Funny you mentioned bore diameters and hard chrome plating. My father was an Olympic shooter and had won a Queen's Medal for Champion Shot as a young soldier. He was a big deal for all things shooting. When the FN C1A1 rifles first appeared, he naturally tested as many as he could find in order to get a very competitive rifle as he went after more Queen's Medals. I remember him telling me that the best rifles were the absolutely earliest serial numbers and first batch of barrels. It seems they had been bored with new sharp tooling and were pretty good as barrels went. Then someone realized they needed to be chrome plated, so these barrels had chrome on top of specified dimensions. They were tight! And shot like crazy! He ended up with 5 Queen's Medals, 1 with a No.4 and 4 with FNs.

[Bob Phillips]

A very interesting story about chrome rifle bores,thanks for sharing. Its funny but now days most of us associate chromed barrels with chinese rifles like the SKS. I don't know if any of our modern sport rifles manufacturers use chrome bores. My understanding about SKS bores is that they are highly resistant to corrosion.
On a slighly related topic I found a video on the net showing the use of a non electrical conducting "arbor" or " twisted core" with wires twisted along it. It is inserted in a barrel blank and electricity applied and it erodes the bore beside the twisted(spiral) electrodes to create etched rifling in the barrel. AN interesting idea, ever heard of this before?

[Perry Kitson]

Quite often standard steel bore cylinders used chromed piston rings, and on the chrome bore cylinders, standard steel rings, no chrome. It takes forever for chrome rings and chromed bores to seat in.
I had a number of interesting conversations with different companies that do the channel chroming on aircraft cylinders regarding the choking of the bore diameter. C4 cylinders were to have .008" to .010" of choke when new, anything less than .004" was NS.
One company in particular said they had done barrels with no choke and up to .010".
Maximum build up of chrome seemed to be .006"(.012" on dia.) for the repairing of used cylinders.

Regarding chromed rifle bores, most competitive shooters now prefer non chromed bores for accuracy. Hard chrome makes cleaning easier, and greatly extends the life of the barrel.

[Bob Phillips]

I wanted to revive this post to ask for some input from our knowledgable membership pool. I have been hunkered down at home (like many of you) for a long time now, and have been using the time to get some longstanding jobs finished. One job has been to continue to sort through and catalogue my collection of engine parts. One item I have found raises questions so here goes;
I have a bunch of sealed ring packages for the 975 that are described (in one book) as being "tinned". They are light coloured and almost look like they have been spray painted with aluminium paint. The coat seems to wear off or rub off relatively easily. I have attached photos where you can see the cast iron base material beneath the coating. These spots are not corrosion.
Does anyone know anything about tinned rings, application, pros and cons?
Any input appreciated.
C4ringsa.jpg

C4ringsb.jpg

RIngs2.jpg

RIngs3.jpg

[David Herbert]

I can only think that the tinning is for quicker bed in or corrosion resistance in storage. I have never heard of this before in any application so, sorry, not much help.

David

[Bob Phillips]

In various previous posts I have talked about chrome vs steel cylinders. One of the big advantages to chrome is their resistance to corrosion in damp or high moisture environments. I have made mention of an engine I opened up to check for damage ( and found smashed pistons, cylinder case etc) and point out that it was equipped with steel cylinders. (first photo)

you can see that the cylinder bore is badly rusted and even in the best of circumstances a steel bored cylinder will likely need some level of reconditioning to be serviceable again. So now check out the next group of photos. They show an engine, (opened up to ensure no catastrophic damage) with chrome bores. This engine was rebuilt in 1945, has been stored outside under cover in the weather extremes of Canada. You can see there is only the spattered beginnings of rust on the chrome, some of which I wiped off with an oily rag. I seems possible that the pistons may be removed without too much difficulty and may even prove to be reusable in a fresh overhaul.
The downside of chrome (as Perry pointed out in post 138) is the rings take much longer to seat and they typically use more oil. However for a tank owner who doesnt have the advantage of a temperature controlled storage environment or who doesn't start the vehicle regularly, chrome may well be the way to go. Overall, a nice engine considering it has been sitting all these years!